Nanofiltration is a pressure-driven membrane separation process between reverse osmosis and ultrafiltration. Nanofiltration membranes have a pore diameter in the order of several nanometers, and are widely used in fresh water softening, seawater softening, potable water purging, water quality improvement, oil-water separation, waste water treatment and reclaim, and fraction, purification and concentration of chemicals such as dyes, antibiotics, polypeptides and polysaccharides.
At present, most of commercial nanofiltration membranes are prepared by utilizing a polysulfone ultrafiltration membrane as a substrate, and interfacially polymerizing a polyamine in water phase and a polyfunctional acyl halide in organic phase on the surface of the polysulfone ultrafiltration membrane, so that the final products are complex nanofiltration membranes. U.S. Pat. No. 4,769,148 and U.S. Pat. No. 4,859,384 disclose such polyamide-based nanofiltration membranes. In addition, U.S. Pat. No. 4,765,897, U.S. Pat. No. 4,812,270, and U.S. Pat. No. 4,824,574 disclose methods for converting a polyamide complex reverse osmosis membrane into a nanofiltration membrane. The moderate stability of polyamides limits the use of polyamide-based nanofiltration membranes to aqueous streams with a pH between 2 and 11.
Recently, many modifications have been made, and many new materials such as sulfonated poly(ether ketone) and sulfonated poly(ether sulfone) have been utilized in the nanofiltration field.
“Acid stable thin-film composite membrane for nanofiltration prepared from naphthalene-1,3,6-trisulfonylchloride (NTSC) and piperazine (PIP)”, J. Membr. Sci., 415-416, 122-131, 2012 reports the interfacial polymerization of polyfunctional sulfonylchloride monomer and piperazine to obtain complex nanofiltration membrane, which is said to maintain stable separation performance in an environment of pH=0.
“Sulfonated poly(etheretherketone) based composite membranes for nanofiltration of acidic and alkaline media”, J. Membr. Sci., 381, 81-89, 2011 reports a nanofiltration membrane having good tolerance to both acids and bases, and “Crosslinking of modified poly(etheretherketone) membranes for use in solvent resistant nanofiltration”, Journal of Membrane Science, 447, 212-221, 2013 reports a nanofiltration membrane having good tolerance to solvents such as isopropanol and acetone.
Because of its good solubility and film forming ability, PVA has been used in nanofiltration and reverse osmosis field. In 1980's, GFT Company used crosslinked PVA (polyvinyl alcohol)/PAN (polyacrylonitrile) complex membrane as a permeation-vaporation membrane in dehydrating ethanol. CN101462024A discloses a complex reverse osmosis membrane comprising crosslinked PVA on a polyamide layer.
CN101732998A discloses a nanofiltration membrane comprising a porous supporting membrane and a coating of crosslinked polyvinyl furfural and the preparation thereof. Silane coupling agents are known in the art, and are used mainly in the adhesive field.